The pelvic diaphragm forms the floor of the pelvis and serves as a bed for the pelvic organs.
The pelvic diaphragm is formed by the union of the levator ani and the coccygeus muscles. A layer of fascia lines the superior and inferior aspects of the pelvic diaphragm (Figures 12-1A). The levator ani muscle consists of three separate muscles: pubococcygeus, puborectalis, and iliococcygeus.
Superior (A) and inferior (B) views of the pelvic diaphragm muscles.
The pelvic diaphragm circumferentially attaches along the pubis, lateral pelvic walls, and coccyx. The rectum pierces the center of the pelvic diaphragm, giving the appearance of a funnel suspended within the pelvis. In addition to the rectum, the urethra and the vagina (in females) and the urethra (in males) pierce the pelvic diaphragm.
Functions of the pelvic diaphragm are as follows:
- Closes the pelvic outlet.
- Supports the abdominopelvic viscera.
- Resists increases in intra-abdominal pressure.
- Controls the openings of the rectum, urethra, and vagina (e.g., helps retain or release feces during a bowel movement).
- Marks the boundary between the rectum and the anal canal.
The complex organization of overlapping muscles and fascia cause the pelvic diaphragm to be susceptible to injury and damage, especially in women. Repetitive stresses, such as those that occur during labor and delivery, can stretch and damage the levator ani muscles and cause pelvic floor insufficiency and its associated clinical problems (e.g., uterine prolapse; urinary incontinence).
Other Muscles of the Pelvic Floor
- Obturator internus muscle. Covers and lines most of the lateral wall of the pelvis. The obturator nerves and vessels and other branches of the internal iliac vessels course along the medial surface of the obturator internus muscle. The obturator internus muscle exits the pelvis through the lesser sciatic foramen and inserts on the greater trochanter of the femur and performs external hip rotation.
- Piriformis muscle. Covers most of the posterior wall of the pelvis. The piriformis muscle exits the pelvis through the greater sciatic foramen and inserts on the greater trochanter of the femur and performs external hip rotation. The sacral plexus of nerves is medial to the piriformis muscle.
The perineum is the diamond-shaped region inferior to the pelvic diaphragm. The pubic symphysis, pubic arches, ischial tuberosities, and coccyx bound the perineum. An imaginary line between the ischial tuberosities divides the perineum into an anterior (urogenital) triangle and a posterior (ischioanal) triangle (Figure 12-2A). The urogenital triangle extends between the paired ischiopubic rami. The ischioanal triangle is the fat-filled area surrounding the anal canal.